Au colloids have been prepared by reduction of metal salt with UV irradiation in the presence of dendrimers. Poly(amidoamine) dendrimers with surface amino groups for generations 0, 1, 2, 3, 4, and 5 (G0-5) have been used. The average particle sizes decrease with an increase of concentration of surface amino group of the dendrimers. In particular, Au colloids with a diameter less than I nm are obtained in the presence of the later generation dendrimers(G3-5) at above the molar ratio of surface amino group of dendrimer/HAuCl4(4:1). These concentrations are extremely small compared with that of other polymers, indicating that the dendrimers have an effective protective action for the formation of Au colloids.